Late Palaeozoic Ultramafic Lavas in Yunnan, SW China, and their Geodynamic Significance

marine sequences within several orogenic belts in YunWe report petrological and geochemical data on ultramafic pillow nan, SW China (Figs 1 and 2; Fang et al., 1996). These lavas from Late Palaeozoic marine sequences in Yunnan, SW lavas have been interpreted to be oceanic plateaux or China. These lavas have >26 wt % MgO, euhedral to subhedral seamount eruptive rocks associated with mantle plumes olivine phenocrysts and acicular or quench clinopyroxene crystals or hotspots in the Palaeo-Tethys ocean basin. The plateau with or without microlitic plagioclase in a devitrified and altered lavas are thought to have been accreted onto the existing glassy matrix. These ultramafic lavas are compositionally komatiitic continents as a result of continental collision during the (>18 wt % MgO), but we term them high-Mg picrites because long and complex evolutionary history of the Tethys they lack spinifex-textured olivine. Although olivines in these picrites ocean basins before the ultimate amalgamation of Eurasia are cumulate crystals, causing the high MgO contents of the bulk and Gondwana in the Tertiary (Fang et al., 1996). Given rocks, the high forsterite content of these olivines, Fo = 0·902 ± the fact that ultramafic lavas (i.e. komatiites or high0·011, suggests that the primitive magmas parental to the picrites magnesian picrites) are very high temperature mantle would have had >17–19% MgO, which is similar to the melts that are common in the Archaean, but rare in the estimates for primary magmas of the Cretaceous Gorgona komatiites Phanerozoic (Arndt & Nisbet, 1982), except for those of in the literature. The calculated liquidus temperature of the primitive Cretaceous age found on Gorgona Island (e.g. Echeverria, magmas parental to the Yunnan picrites is >1400 ± 25°C, 1982; Kerr et al., 1996; Arndt et al., 1997; Révillon et al., which would imply a mantle potential temperature of >1540 ± 2000), it is important that these Late Palaeozoic ultramafic 30°C. This is inconsistent with magma generation beneath ocean lavas in SW China are carefully examined. The ultramafic ridges or in the mantle wedge above a subduction zone, but is lavas appear to have been erupted under submarine consistent with a mantle plume origin, which is fully supported by conditions and exhibit well-preserved pillow structures the trace element characteristics of the magmas. (Fig. 3). Despite the widespread occurrence of the picrites (Fig. 1), the outcrops are rather sparse because of neotectonic complications (e.g. effects of India–Eurasia collision and the uplift of the Tibetan plateau) and because